Breast cancer vaccine works better with letrozole than with tamoxifen

A vaccine that targets cancer cells significantly increased survival in female mice with breast cancer when administered with the hormonal therapy letrozole, but not with the hormonal therapy tamoxifen.

The vaccine, known as L-BLP25 (Stimuvax), specifically targets Mucin1 glycoprotein, an antigen expressed in an altered form by cancer cells. The vaccine prompts T-lymphocytes to destroy the cancer cells.

In a recent study described in Clinical Cancer Research (2012;18[10]:1-11), mice were injected weekly with the vaccine or with a placebo for 8 weeks. Some mice also received daily doses of either letrozole 0.8 mg/kg or tamoxifen 50 mg/kg. Both of these estrogen-blocking hormones are commonly used in the treatment of breast cancer.

Daily oral doses of either one of the hormones without vaccine resulted in a significant survival advantage compared with untreated mice, and vaccinated mice exhibited an immune response with or without the addition of letrozole or tamoxifen. Yet despite the similar actions of the two agents, only letrozole plus vaccine conferred a statistically significant survival benefit; tamoxifen plus vaccine did not.

“Hormonal drugs affect the immune system in different ways, and the actions of tamoxifen prevent the vaccine from working effectively,” affirmed the study's principal investigator, Michael DeGregorio, of the Division of Hematology and Oncology at University of California (UC), Davis, in a statement issued by the UC Davis Health System. “This highlights the importance of rigorous testing of different combinations of therapies before using them in patients.”

The researchers also learned that the vaccine worked best when tumor burden (the amount of cancer present) was low. This suggests that the vaccine may one day be used as a preventive measure for women at high risk of developing breast cancer or for the treatment of early-stage disease.